1. Field of the Invention
The present invention relates to a process for carrying out a thermal-etching treatment of a compound semiconductor substrate to be used in molecular beam epitaxy and an apparatus for performing same. More particularly, it relates to a process for the thermal removal of surface carbon or carbide from a GaAs substrate which is subject to a molecular beam epitaxy and an apparatus for performing same.
2. Description of the Related Art
A molecular beam epitaxy (MBE) technique for compound semiconductors such as GaAs is often used for the fabrication of high speed devices such as high electron mobility transistors (HEMT), and it is well known that a carrier depletion layer is formed around the interrupted growth interface and the substrate-epitaxial interface as a result of this technique. Such a depletion layer will sometimes degrade the performance of the devices.
Therefore, the cleaning process of the substrate in the MBE chamber prior to the epitaxial growth process is very important to an improvement of the quality of the epitaxial films. The major contaminants formed GaAs on the GaAs substrate after chemical etching and loading into an MBE chamber are oxygen and carbon in the form of carbon dioxide (CO.sub.2) or carbon hydride (C.sub.n H.sub.2n+1) etc., (cf: Interface and doping profile characteristics with molecular-beam epitaxy of GaAs:GaAs voltage character, J. Appl. Phys. 45(4) April, 1974). Oxygen can be thermally removed by heating the substrates to .about.530.degree. C. However, this heating process is not enough to eliminate other forms of contamination, especially carbon or carbide, from the GaAs substrate. The conventional treatment .about.530.degree. C. Further, when a semiconductor crystalline layer is epitaxially formed on a surface of a GaAs substrate in a state wherein carbon atoms, etc.,are adhered thereto, the carbon atoms, etc., act as acceptors so that an interfacial layer between the GaAs substrate and the semiconductor crystalline layer is formed. In order to negate the effect of the formation of the interfacial layer, the formation of a thick buffer layer is necessary. This takes a lot of time and thus the production output time is decreased.